1. Technical Field of the Invention
The present invention relates generally to the wireless telecommunications field and, in particular, to a private radio communications system that is compatible for use with cellular mobile telephones.
2. Description of Related Art
The commercial utilization of digital mobile communications systems for public telephony has grown tremendously during the past decade. For example, since the mid-1980's, a large number of digital cellular communications systems have been fielded for public use throughout the world, such as the Global System for Mobile Communications (GSM) in Europe, the Personal Digital Cellular (PDC) System in Japan, and the Digital Advanced Mobile Phone System (DAMPS) in North America. However, although many of the technical specifications for these systems are different, the GSM air interface standard, which was created in Europe, is being used in many of the other cellular systems deployed throughout the world.
The GSM air interface standard, which is so widely in use, is based on a Time Division Multiple Access (TDMA) scheme of channel allocation. This TDMA allocation scheme allows the use of a hierarchical cell communications structure, in which so-called macrocells cover large regional areas or districts, microcells cover somewhat smaller areas (e.g., a stretch of highway), and picocells cover even smaller areas (e.g., the size of one or two rooms). However, an important requirement of existing systems using such a hierarchical cell structure is that the base stations deployed, which define the cell coverage areas, must all belong to the same cellular network (e.g., Public Land Mobile Network or PLMN).
In business and residential environments, users of telephony services have limited mobility and, therefore, can be offered services not normally available to the highly mobile users of cellular networks. For example, U.S. Pat. Nos. 5,428,668 and 5,535,259 disclose a private radio base station, which is not an integral part of a disclosed cellular network, but is connected directly to a Public Switched Telephone Network (PSTN). When a mobile cellular terminal comes within radio range of a "home base station" (HBS) of the private system, the mobile terminal is converted operationally to a cordless phone. Using this cordless mode of operation, a mobile user can realize numerous advantages over the cellular mode, such as the lower PSTN rates incurred, higher quality voice communications, and a substantially longer standby time.
Many similar and other advantages would be realized if a private radio system were to have an air interface that is compatible with a cellular network air interface. For example, a mobile terminal could be converted into a dual-mode cellular-HBS portable terminal, by downloading software from the mobile network or private system to implement the cordless mode of operation. Additionally, the private radio system's existing base station hardware could be used in such a dual-mode cellular-HBS operation, provided the base station's transmit and receive frequencies are exchanged. Nevertheless, these private radio systems are not operationally compatible with existing cellular networks, and the above-described advantages are not being realized.